1. Field of the Present Invention
The present invention relates generally to the field of gas detectors, and, in particular, to the art of supplying sample air to one or more heated electrode sensing devices to detect halogenated refrigerants.
2. Background Art
Electronic refrigerant leak detectors typically include a power supply, such as a replaceable or rechargeable battery, one or more sensing devices (sensors), a printed circuit board and a sample path assembly for drawing air into or across the sensing device. The sample path of most electronic refrigerant leak detectors start at a flexible hollow tube of varying length called a goose-neck probe. The free end of the probe is positioned where the operator wants to take an air sample. The sample of air is drawn into the free end of the probe, through a filter, and then across or past the sensing device before being exhausted from the detector. The detector generally has some means for drawing the sample of air along the sample path, for example a fan or pump.
Currently available electronic refrigerant leak detectors include the xe2x80x9cD-TEK,xe2x80x9d manufactured by Leybold-Inficon, headquartered in East Syracuse, N.Y.; xe2x80x9cThe Informant,xe2x80x9d manufactured by Bacharach, headquartered in Pittsburgh, Pa.; and the H10Xpro xe2x80x9cTop Gun,xe2x80x9d manufactured by Yokogawa Corporation of America, headquartered in Newnan, Ga. Each detector has a gooseneck probe extending some length from the detector body and a means for drawing an air sample through the probe. The air sample is drawn through or across the sensor which is electronically connected to a printed circuit board that is disposed within the interior of the detector body.
The D-TEK uses a centrifugal fan surrounded by a shroud to draw air through a probe. One end of the flexible goose-neck probe is glued to the shroud while the free end extends about 16 inches from the detector body. A sensor is located within the free end of the flexible probe. The fan draws the air sample through the free end of the probe, across the sensor, through the remaining length of the probe and into the shroud before it is exhausted from the fan. Signals from the sensor are transmitted to the printed circuit board via electrical wires inserted into the probe and traveling the length of the probe from the sensor to the printed circuit board. The wires are inserted through a rubber plug that seals the shroud where the probe is attached so that leaks around the wires are minimized. A probe tip, attached to the free end of the flexible probe, encloses the sensor. The sample flow rate across the sensor is approximately 35 standard cubic centimeters per minute (SCCM).
Unfortunately, this construction has several disadvantages concerning the manufacture and use of the detector. The relatively low flow rate across the sensor results in a low sensor sensitivity, a longer response time and a longer clearing time (the amount of time required to purge the sample path assembly and the sensor of previously analyzed gas so that a new sample can be taken and analyzed). The flexible probe is glued to the fan shroud, which makes the replacement of a damaged flexible probe difficult and time consuming. The wires connecting the sensor to the printed circuit board are inside the flexible probe, and hence, obstructs the flow path between the probe tip and the fan, potentially resulting in an unquantifiable and unpredictable resistance to the flow of the air sample through the flexible probe, ultimately causing an inconsistent sample flow across the sensor from one use to the next and from one detector to the next. Besides obstructing the flow path, the length of the wires connecting the sensor and the printed circuit board increases the electrical resistance of the wires, increasing demand on the battery and reducing the operating time of the detector without changing batteries or recharging the existing battery. Finally, inserting wires through the small diameter flexible tubing of the flexible probe, sealing the flow path around the wires, and gluing the flexible probe to the fan shroud all increase the difficulty and time required to manufacture the sample path assembly of the detector and to replace the probe or fan if either are damaged.
Like the D-TEK, xe2x80x9cThe Informantxe2x80x9d uses a fan surrounded by a shroud to draw air through a 20 inch flexible probe. One end of the flexible probe is glued to the shroud and the free end is covered by a probe tip. The sensor is located within free end of the flexible probe. The sensor is covered by the probe tip and a filter. Wires connecting the sensor to the printed circuit board are routed through the shroud and into the interior of the flexible probe. A flexible sealant is used to seal the shroud and flexible probe around the wires. The typical flow rate is approximately 50 SCCM. The Informant has many of the same disadvantages as the D-TEK. In addition, the use of a flexible sealant increases the time for manufacture because the sealant must be xe2x80x9ccuredxe2x80x9d to create a usable seal.
The xe2x80x9cTop Gunxe2x80x9d detector offers a different approach to constructing a sample path assembly for an electronic refrigerant leak detector. The sensor is connected directly (i.e., is soldered) to the printed circuit board, eliminating the wires found in the flow paths of the D-TEK and The Informant. A flexible probe is approximately 16 inches long and is removably attached to the detector body. A rotary vane pump draws air through the flexible probe at a flow rate of approximately 250 SCCM. The air sample travels through the flexible probe, into the inlet of the rotary vane pump, through the pump to the outlet of the pump, and through a xe2x80x98Txe2x80x99 split before encountering the sensor. The air sample from the outlet of the pump is split into two paths at the xe2x80x98Txe2x80x99 splitxe2x80x94one path is exhausted from the detector and one path continues to the sensor. Thus, while the flow rate through the flexible probe and rotary vane pump is about 250 SCCM, the actual flow rate of the air sample across the sensor is considerably less and is approximately equivalent to the flow rate of the air sample across the sensors in the D-TEK and The Informant.
The removable flexible probe of the Top Gun, which makes replacement easy, is advantageous over the D-TEK and the Informant. Furthermore, unlike the D-TEK and The Informant, there are no wires traveling the length of the probe to connect the sensor to the printed circuit board. This results in a reduced demand on the battery and an unobstructed flow through the flexible probe. Unfortunately, however, the construction of the sample flow path assembly of the Top Gun presents other disadvantages, most notably the inaccessibility of the sensor, which is fixedly attached to the printed circuit board located inside the detector. This inaccessibility makes replacement of the sensor extremely difficult. Another disadvantage is the placement of the sensor on the outlet side of the pump which introduces potential mixing problems associated with the air sample within the pump, and an increase in the clearing time. Further, because of the flow split, the increased flow rate through the flexible probe results in an increased demand on the battery by the pump without an appreciable increase in sensor sensitivity. The reduced flow rate across the sensor is a necessary component of the xe2x80x9cTop Gunxe2x80x9d design. Otherwise, the sensor may be damaged if subjected to the full, high, flow rate. Finally, the additional tubing within the detector body required for the xe2x80x98Txe2x80x99 split increases the number of steps needed to manufacture the flow path assembly, thereby increasing the difficulty and time required.
For greatest detector efficiency, the sensitivity of the sensing device (sensor) must be maximized to an optimum level, the response time of the sensing device should be minimized, and the time needed to clear the detector of already sampled gas (the clearing time) must be minimized, all while maintaining a reasonable demand on the power supply (usually measured in terms of battery life). One way to maximize the sensitivity of the sensing device is to increase the flow rate of the air sample flowing across or past the sensing device. The clearing time can be minimized by shortening the sample path between the probe tip and the sensing device and/or increasing the flow rate so the sampled air is exhausted from the detector more quickly. The response time may be minimized by increasing the flow rate of the air sample or shortening the signal path between the sensing device and the electronic circuit. Battery life may be maintained in a variety of ways, including shortening the signal path between the sensing device and electronic circuit, thereby reducing the resistance losses of the wires connecting the sensing device to the electronic circuit, or by reducing the demand placed on the battery by the pump or the fan, either by operating at a lower flow rate or providing a more efficient sample path.
Thus, mindful of the disadvantages of many of the currently available electronic refrigerant leak detectors, a need exists for a gas leak detector that has a sample flow path that maximizes sensing device sensitivity, minimizes respond time, and minimizes clearing time, all while maximizing battery life and reducing manufacturing difficulty and time.
Briefly summarized, the present invention relates to a sample flow path assembly for use in a gas detector that is capable of sensing the presence of at least one predetermined gas and that has a temperature controlled sensing device, a bias current controlled sensing device, or a combination thereof. The sample flow path assembly includes a means for generating a sample air flow past the sensing device at a flow rate in excess of about 300 SCCM and means for conducting the sample air flow past the sensing device.
The means for generating a sample air flow is a pump that has an inlet port and an outlet port. The pump is located within a body of the gas detector. The means for conducting the sample air flow past the sensing device includes a socket connected to the inlet port of the pump, a collar located at one end of the housing and surrounding the socket, and a probe attached to the collar whereby the probe extends beyond the housing of the detector to open to the surrounding environment. The sensing device is disposed upon and supported by the socket such that the sensing device is disposed between the probe and the pump. A flexible interconnect connects the sensing device to a printed circuit board. The pump motor may be electrically connected to the printed circuit board and supplied DC power, preferably by a battery.